1. Field of the Invention
The present invention relates to power supply circuitry and, in particular, to a low noise, low voltage drop, active power supply filter suitable for monolithic fabrication.
2. Description of the Prior Art
In conventional communication circuitry it is often desirable to employ a power supply ripple filter which will provide a well filtered, low noise supply voltage capable of high output current and which drops as small a voltage as possible across the filter. Ripple filters known in the prior art do not maintain a minimum voltage drop nor minimal noise levels. Further, prior art circuits require relatively large capacitance values to achieve a desired cut-off frequency and are not highly suitable for monolithic integration.
Accordingly, it is an object of the invention to provide a low noise active power supply ripple filter capable of high output currents while maintaining low voltage drop across the filter.
It is another object of the invention to provide a low noise active power supply ripple filter which minimizes the capacitance values required to achieve desired frequency rejection characteristics.
It is yet another object of the invention to provide a low noise active power supply ripple filter which is particularly suitable for monolithic integration.
Briefly, in accordance with the invention a low noise active power supply filter capable of high output current is provided for eliminating alternating current components from a direct current voltage, with a minimal input-output voltage drop. A control device such as a bipolar transistor is utilized with an input terminal coupled to the power supply and an output terminal coupled to a load. The control device controls and therefore to the power supply the current from the input terminal to the output terminal via a signal coupled to a control terminal. A reference circuit coupled to the input terminal of the control device provides a reference voltage which approximately tracks the power supply input voltage. An amplifier having its output coupled to the control terminal of the control device amplifies a signal applied across first and second input terminals. A filter is coupled to the reference means to low pass filter the reference voltage and then couple the filtered reference voltage to the first input of the amplifier means. A feedback loop couples the control means output terminal to the second input of the amplifier means.
The invention herein disclosed provides a low noise active power supply ripple filter capable of high output current which tracks the direct current supply voltage thereby maintaining a minimum voltage drop across the filter and low power dissipation. The filter also provides some temperature compensation, permits the use of small value filter capacitors and is particularly suitable for monolithic fabrication.